Handheld electrochemical sensor

ABSTRACT

A handheld sensor device is provided for measuring an ion concentration in a solution. The solution is in an electrochemical cell that includes a counter electrode, a working electrode, and a reference electrode. The sensor includes a control amplifier configured to provide a current through the counter electrode and the working electrode so as to maintain a predetermined voltage between the working electrode and the reference electrode. The sensor also includes a current amplifier configured to measure the current provided through the counter electrode and the working electrode. In one embodiment, the sensor also includes a direct digital frequency synthesizer (DDFS) including a phase accumulator. The DDFS is configured to selectively generate a waveform specified by an electrochemical technique such as square wave voltammetry, cyclic voltammetry, linear sweep voltammetry, differential-pulse polarography, normal-pulse polarography, or other known electrochemical techniques.

RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 11/761,916 filed Jun. 12, 2007, entitled “Electrochemical Chipwith Miniaturized Sensor Array” which claims the benefit of U.S.Provisional Application No. 60/813,299, filed Jun. 12, 2006, which ishereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

This disclosure relates generally to electrochemical analysis. Morespecifically, this disclosure relates to a portable, handheldelectrochemical sensor device including a waveform generator integratedwith potentiostat. This disclosure also relates to a microelectrodearray useable with the handheld electrochemical sensor device.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the disclosure aredescribed, including various embodiments of the disclosure withreference to the figures in which:

FIG. 1 is a block diagram of a conventional electrochemical cell setup;

FIG. 2 is a block diagram of an example electrochemical chip accordingto one embodiment;

FIG. 3 schematically illustrates an example electric potential waveformsuitable for a square wave (SW) voltammetry technique according to oneembodiment;

FIG. 4 is a schematic diagram of an RC and Schmitt circuit and resultingwaveforms according to one embodiment;

FIG. 5 is a schematic diagram of a summing amplifier circuit forgenerating a waveform according to one embodiment;

FIG. 6 is a partial schematic diagram and partial block diagram of an RCand Schmitt circuit with a constant current source according to oneembodiment;

FIG. 7 is a schematic diagram of a SW function generator circuit thatincludes RC and Schmitt circuits with constant current sources and asumming amplifier circuit according to one embodiment;

FIG. 8 schematically illustrates an example waveform generated by the SWfunction generator circuit shown in FIG. 7 according to one embodiment;

FIG. 9 is a block diagram of a direct digital frequency synthesizer(DDFS) configured to generate a user-defined ultra-low frequencywaveform according to one embodiment;

FIG. 10 is a block diagram of an example DDFS circuit according to oneembodiment;

FIG. 11 schematically illustrates an example waveform generated by theexample DDFS circuit shown in FIG. 10 according to one embodiment;

FIGS. 12-13 schematically illustrate enlarged versions of the waveformshown in FIG. 11 to provide additional details;

FIG. 14 is a schematic diagram of the example DDFS circuit shown in FIG.10 according to one embodiment;

FIG. 15 is a block diagram of a control amplifier according to oneembodiment;

FIG. 16 is a block diagram of a control amplifier according to anotherembodiment;

FIG. 17 is a schematic diagram of an example control amplifier circuitaccording to one embodiment;

FIG. 18 is a block diagram of a basic measuring circuit according to oneembodiment;

FIG. 19 is a block diagram of a measuring circuit including a voltagemeter and a floating power supply according to one embodiment;

FIG. 20 is a block diagram of a measuring circuit that provides anactive current sink according to one embodiment;

FIG. 21 is a schematic diagram of a circuit without guarding used tomeasure a current according to one embodiment;

FIG. 22 is a schematic diagram of a circuit with guarding used tomeasure a current according to one embodiment;

FIG. 23 is a schematic diagram of an example current amplifier circuitaccording to one embodiment;

FIG. 24 is a block diagram of a lock-in amplifier according to oneembodiment;

FIG. 25 is a schematic diagram of an example lock-in amplifier circuitaccording to one embodiment;

FIG. 26 is a block diagram of a measurement and control unit shown inFIG. 2 according to one embodiment;

FIG. 27A schematically illustrates square wave voltammetry (SWV)responses to various K₃Fe(CN)₆ solutions using a conventionalelectrochemical instrument;

FIG. 27B schematically illustrates SWV responses to the same K₃Fe(CN)₆solutions discussed with respect to FIG. 27A using the electrochemicalchip disclosed herein;

FIG. 27C schematically illustrates data combined from FIGS. 27A and 27Bto further illustrate the consistency of responses from conventionalelectrochemical instruments and the systems and methods disclosedherein;

FIGS. 28A-28B are schematic diagrams of nine microelectrodes arranged inrespective half arrays according to certain embodiments;

FIG. 29 is a schematic diagram of a completed array that illustratesconnection pads and interconnection traces for a first half array ofnine microelectrodes and a second half array of nine microelectrodesaccording to one embodiment;

FIG. 30 is a schematic diagram of various steps in a process formanufacturing the microelectrode array shown in FIG. 29 usingphotolithography according to one embodiment;

FIG. 31 is a schematic diagram of a finished glass slide with individualmicroelectrodes comprising three arrays respectively corresponding tothree wells according to one embodiment;

FIGS. 32-33 graphically illustrate results of testing microelectrodesdisclosed herein in Potassium Ferric Cyanide (K₃Fe(CN)₆) using cyclicvoltammetry;

FIGS. 34-36 graphically illustrate results of testing themicroelectrodes disclosed herein in Pb ions using anodic strippingvoltammetry (ASV);

FIGS. 37-39 graphically illustrate results of testing themicroelectrodes disclosed herein in Zn ions using ASV; and

FIG. 40 graphically illustrates results of testing the microelectrodesdisclosed herein in Cu ions using ASV.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Electrochemistry is especially useful in a variety of areas such ascorrosion, coatings, electroplating and fuel cells. Through theknowledge of the electronic and electrical aspects of chemicalreactions, and the investigation of the transfer of electrons, a currentmay be induced and measured in an electrolyte. The magnitude of thisinduced current offers information about the concentration of variousions in an aqueous solutions.

As discussed below, electrochemical cells are generally used to induceand measure the current in the electrolyte. At a negative electrode ofthe electrochemical cell, electrons are given off and “oxidation” takesplace. At a positive electrode of the electrochemical cell, the excesselectrons are collected and “reduction” occurs. This process of givingand taking of electrons creates an electric current (ion current). Theion current, which may be in a range of, for example, several PicoAmperes (pAs), may be used to determine ion concentrations. Thus, forexample, water quality may be monitored and controlled by measuring thedensities of ions such as Chloride, Sulfate, Magnesium, Calcium, Sodium,Bicarbonate, and Silica in rivers, lakes and other water bodies.Electrochemical cells may also be used, for example, in the biomedicalfield to analyze ion concentrations in bodily fluids. Of course, manyother electrochemical analysis applications will occur to those skilledin the art.

In one embodiment, a handheld sensor device for measuring an ionconcentration in a solution is provided. The solution is in anelectrochemical cell that includes a counter electrode, a workingelectrode, and a reference electrode. The sensor device includes acontrol amplifier configured to provide a current through the counterelectrode and the working electrode so as to maintain a predeterminedvoltage between the working electrode and the reference electrode. Thesensor also includes a current amplifier configured to measure thecurrent provided through the counter electrode and the workingelectrode.

In one embodiment, the sensor also includes a direct digital frequencysynthesizer (DDFS) including a phase accumulator. The DDFS is configuredto selectively generate a waveform specified by an electrochemicaltechnique. In one such embodiment, the DDFS is configured, for example,to generate a waveform suitable for square wave voltammetry. Such awaveform may comprises a square wave of constant magnitude superimposedon a triangle wave. In one embodiment, the square wave has a frequencyof approximately 15 Hz and the triangle wave has a frequency ofapproximately 0.1 Hz. However, in other embodiments, the square wave andthe triangle waves have respective frequencies in a range betweenapproximately 0.02 Hz and approximately 20 MHz. In other embodiments,the electrochemical technique may be cyclic voltammetry, linear sweepvoltammetry, differential-pulse voltammetry, normal-pulse voltammetry,or other known electrochemical techniques.

In one embodiment, the current amplifier includes an electricallyconductive guard configured to reduce a portion of the current measuredby the current amplifier that is attributable to a leakage currentthrough the reference electrode. In addition, or in another embodiment,the current amplifier may be configured to measure the currentthroughout an entire range comprising approximately 100 pA toapproximately 2 mA.

In one embodiment, the handheld sensor also includes a lock-in amplifierincluding a phase sensitive detector. The lock-in amplifier isconfigured to reduce the measured current's noise.

In one embodiment, the handheld sensor is used with a microelectrodearray formed on a glass substrate. In one such embodiment, themicroelectrode array includes at least one microelectrode that isapproximately 10 μm wide or less.

The embodiments of the disclosure will be best understood by referenceto the drawings, wherein like elements are designated by like numeralsthroughout. In the following description, numerous specific details areprovided for a thorough understanding of the embodiments describedherein. However, those of skill in the art will recognize that one ormore of the specific details may be omitted, or other methods,components, or materials may be used. In some cases, operations areneither shown nor described in detail.

Furthermore, the described features, operations, or characteristics maybe combined in any suitable manner in one or more embodiments. It willalso be readily understood that the order of the steps or actions of themethods described in connection with the embodiments disclosed may bechanged as would be apparent to those skilled in the art. Thus, anyorder in the drawings or Detailed Description is for illustrativepurposes only and is not meant to imply a required order, unlessspecified to require an order.

I. SYSTEM OVERVIEW

FIG. 1 is a block diagram of a conventional electrochemical cell 100used to measure an induced current. The magnitude of this inducedcurrent offers information about the concentration of various ions in anaqueous solution 110. The electrochemical cell 100 generally includes acounter electrode 112, a reference electrode 114, and a workingelectrode 116 immersed in the ionic solution 110.

Although not shown, electrochemical cells 100 generally include apotentiostat that controls the voltage difference between the workingelectrode 116 and the reference electrode 114 so as to maintain thevoltage difference at a constant preset value. In practice, thepotentiostat records potential differences between the working electrode116 and the reference electrode 114 without polarizing the referenceelectrode 114. The potentiostat also compares the potential differenceto the preset voltage and forces a current to go through the counterelectrode 112 towards the working electrode 116 to counteract thevoltage difference between the working electrode 116 and the referenceelectrode 114. According to the embodiments discussed herein, thepotentiostat is configured to perform an accurate and stable generationand measurement of direct current (DC) voltages and currents.

Generally, there are several known electrochemical techniques such ascyclic voltammetry, linear sweep voltammetry, differential-pulsevoltammetry, normal-pulse voltammetry and square wave voltammetry (SWV).SWV, for example, uses a square wave of constant amplitude superimposedon a staircase wave form. The current is measured at the end of eachhalf-cycle. The current measured in the reverse half-cycle is subtractedfrom the current measured on the forward half-cycle. This currentdifference may be plotted as a function of the applied potential.

In certain example embodiments disclosed herein, an electrochemical chipis configured to use SWV. However, an artisan will understand from thedisclosure herein that other electrochemical techniques may also beused. For example, in one embodiment, a user may selectively configurethe electrochemical chip to use any particular electrochemicaltechnique.

FIG. 2 is a block diagram of an example electrochemical sensor device200 according to one embodiment. In one embodiment, the electrochemicalsensor device 200 is a computerized hand-held test station on a printedcircuit board. In another embodiment, the electrochemical sensor device200 is integrated on a single chip. Thus, the electrochemical sensordevice 200 may also be referred to herein as “electrochemical chip 200.”The electrochemical chip 200 provides high accuracy and flexibility. Theelectrochemical chip 200 is also low cost as compared to conventionalelectrochemical systems.

The electrochemical chip 200 includes a measurement and control (MAC)unit 210 configured to manage data acquisition and potentiostatfunctionality. The MAC unit 210 may include, for example, a processor(e.g., an 8-bit Microcontroller Unit (MCU) or other processor) and ananalog-to-digital (A/D) converter. The MAC unit 210 may also include aninterface (e.g., RS-232, universal serial bus (USB), or other computerinterface) configured to provide communication with a host computer (notshown) through which a user may define an electrochemical technique tobe applied and monitor results through a graphical user interface (GUI).Thus, the electrochemical chip 200 may be configured to sample apotential-current wave and process the potential-current wave fordisplay by a host computer.

The electrochemical chip 200 also includes a digital function generator212 integrated with a potentiostat 213 in a single, portable sensordevice. The potentiostat 213 includes a control amplifier 214, apotential amplifier 216, a current amplifier 218, a reference oscillator220, and a lock-in amplifier 222. The digital function generator 212 isconfigured to generate (for this example) an SWV signal. The controlamplifier 214 is electrically connected to the electrochemical cell 100through the counter electrode 112 (CE). As discussed in detail below,the control amplifier 214 is also configured to integrate the functionsof a signal amplifier, a power amplifier, and a low-pass filter. Thepotential amplifier 216 is electrically connected to the electrochemicalcell 100 through the reference electrode 114 (RE) and includes a voltagefollower with high input impedance.

The current amplifier 218 is electrically connected to theelectrochemical cell 100 through the working electrode 114 (WE) and isconfigured to amplify ultra-low current. Generally, investigation inmaterial science includes measuring ultra-low current in a range betweenapproximately 100 pA and approximately 1 mA. The dynamic range of a goodpotentiostat spans up to 4 decades. The reference oscillator 220 mayinclude a high accuracy, low frequency oscillator and may be derivedfrom, for example, the MCU using programming and an on-chip timer.

The lock-in amplifier 222 is configured to separate a small, narrow bandsignal from interfering noise. As discussed below, the lock-in amplifiermay include, for example, a narrow band pass filter to remove much ofthe unwanted noise while allowing the signal to be measured.

Various example embodiments of the electrochemical chip 200 and/or itsvarious components are provided below. An artisan will recognize fromthe disclosure herein that the example embodiments may be combined invarious ways and that the generalized interconnections betweencomponents illustrated in FIG. 2 may not apply to every specific examplediscussed herein.

II. EXAMPLE FUNCTION GENERATOR EMBODIMENTS

As discussed above, the function generator 212 (according to theexamples discussed herein) is configured to generate a SWV signal at thecounter electrode 112. FIG. 3 schematically illustrates an exampleelectric potential (E) waveform 300 suitable for a square wave (SW)voltammetry technique according to one embodiment. The waveform 300provided to the counter electrode 112 comprises a square wave ofconstant amplitude superimposed on a stair case waveform. As discussedabove, the current of the waveform is measure at the end of eachhalf-cycle and the current measured in the reversed half cycle (see I(2)in FIG. 3) is subtracted from the current measured on the forwardhalf-cycle (see I(1) in FIG. 3). This current difference may bedisplayed as a function of the applied potential.

The waveform 300 shown in FIG. 3 is defined by various parameters suchas initial E, final E, incremental E, SW amplitude, SW frequency(1/period), and scan period. Table 1 below provides example values forthe waveform 300 according to one embodiment. Note that in theseexamples, the waveform 300 may have a positive slope (rising staircase)or a negative slope (descending staircase).

TABLE 1 Example Example Parameter Approximate Value Approximate RangeUnits SW Amplitude 0.025 0.0006 to 2.5 Volts SW Frequency 15.0015  0.02to 20 × 10⁶ Hertz Incremental E 0.004 0.0006 to 2.5 Volts Initial E 0.5   −5 to +5 Volts Final E −0.1    −5 to +5 Volts

From the example approximate values shown in Table 1, the scan periodmay be calculated as: Scan Period=(Init E−Final E)/Incr. E*(1/SWFreq)=9.999 seconds.

The waveform 300 shown in FIG. 3 (or similar waveforms) may be generatedin various ways, including using both analog and digital circuitry. Forexample, the waveform 300 may be generated by an RC and Schmitt circuit,a summing amplifier, an RC and Schmitt circuit with a constant currentsource, or a direct digital frequency synthesizer. Details for suchexample embodiments, along with some discussion of correspondingadvantages and disadvantages, are provided below.

A. RC and Schmitt Circuit

The SW function generator 212 may be constructed by adding a resistorand a capacitor to an inverting Schmitt trigger circuit. For example,FIG. 4 is a schematic diagram of an RC and Schmitt circuit 400 andresulting waveforms 410, 412 according to one embodiment. The Schmitttrigger portion of the circuit 400 includes an operational amplifier 414together with two resistors R2, R3. However, in this embodiment, acapacitor C1 controls the voltage at the Schmitt input and a resistor R1charges and discharges the capacitor C1 from the Schmitt output Vo.

The RC and Schmitt circuit 400 shown in FIG. 4 may not providesufficient linearity for certain applications. Thus, it may be necessaryto set an upper tuning point (UTP) and a lower tuning point (LTP) thatare much smaller than the operational amplifier's output levels and tokeep the voltage drop across the resistor R1 approximately constant.Thus, the capacitor charging current is maintained at a substantiallyconstant value.

In one embodiment, the SW function generator 212 also includes a summingamplifier circuit configured to combine, for example, a square wave anda triangle wave to produce a final waveform such as the waveform 300shown in FIG. 3. For example, FIG. 5 is a schematic diagram of a summingamplifier circuit 500 for generating a waveform according to oneembodiment. The summing amplifier circuit 500 receives two or moreinputs (V1, V2, V3) and provides an output voltage that is proportionalto the negative of the algebraic sum of its input voltages asrepresented by the following equation:

$V_{out} = {{- \left( {\frac{V_{1}}{R} + \frac{V_{2}}{R} + \frac{V_{3}}{R}} \right)}{R.}}$

If all resistors shown in FIG. 5 have the same value, thenV_(out)=−(V₁+V₂+V₃).

An advantage of the summing amplifier circuit 500 is that threeoperational amplifiers (e.g., one used to produce a square wave, oneused to produce a triangle wave, and an operational amplifier 510 usedfor the summing amplifier circuit 500) and several resistors andcapacitors are enough to provide a desired waveform. A disadvantage,however, is poor accuracy and linearity. When frequency is low,linearity begins to deteriorate. For example, when the frequency is aslow as approximately 100 Hz, the linearity may not be acceptable forcertain applications.

B. RC and Schmitt Circuit with Constant Current Source

In another embodiment, the function generator 212 also includes aconstant current source to improve the linearity of a capacitor voltage.For example, FIG. 6 is a partial schematic diagram and partial blockdiagram of an RC and Schmitt circuit 600 with a constant current sourceaccording to one embodiment. The circuit 600 includes a Schmitt trigger610 (e.g., the operational amplifier 414 and resistors R2, R3 shown inFIG. 4) electrically coupled to a constant current source 612, acapacitor C2, and a buffer amplifier 614. Although not shown, the outputof the Schmitt trigger 610 may also be electrically coupled to anadditional buffer amplifier so as to provide a square wave. With thehelp of the constant current source 612, the circuit 600 provides ahighly linear triangle wave output having a highly stable centerfrequency at a wide range of operating voltages.

FIG. 7 is a schematic diagram of a SW function generator circuit 700that includes RC and Schmitt circuits 710, 712 with constant currentsources, and a summing amplifier circuit 714 according to oneembodiment. The circuit 700 provides simplicity and is low cost. The RCand Schmitt circuits 710, 712 with constant current sources may include,for example, the circuit 600 shown in FIG. 6. In one embodiment, the RCand Schmitt circuits 710, 712 with constant current sources eachcomprise an NE566 function generator available from PhilipsSemiconductor Linear Products, which is a voltage-controlled oscillatorof exceptional linearity with buffered square wave and triangle waveoutputs.

The frequency of oscillation of the NE566 function generator isdetermined by external resistors and capacitors and a voltage applied toa control terminal (VIN). A resistor R4 may be adjusted to change thefrequency of the triangle wave, a resistor R5 may be adjusted to changethe slope of the triangle wave, a resistor R6 may be adjusted to changethe frequency of the square wave, a resistor R7 may be adjusted tochange the amplitude of the square wave, and a resistor R8 may beadjusted to change the DC potential of whole wave (e.g., at the outputof the summing amplifier 714).

Using an internal constant current source, the NE566 provides acceptablelinearity within a working triangle wave frequencies ranging betweenapproximately 3 Hz and approximately 1 MHz. When the frequency of thetriangle wave is as low as approximately 0.5 Hz, the linearity may notbe acceptable for certain applications. In one embodiment, for example,the SWV specification is approximately 0.1 Hz. In such an embodiment,the frequency is beyond the ability of the NE566's internal constantcurrent source circuit. However, the circuit 700 using the NE566 withthe constant current source circuit may be utilized in some applicationswhere, for example, electrochemical reactions are finished within ashort time period.

FIG. 8 schematically illustrates an example waveform 800 generated bythe SW function generator circuit 700 shown in FIG. 7 according to oneembodiment. In this example, the waveform 800 is synthesized bycombining a triangle wave having a frequency of approximately 10 Hz anda square wave having a frequency of approximately 1000 Hz.

C. Direct Digital Frequency Synthesizer (DDFS)

In another embodiment, the function generator 212 includes a directdigital frequency synthesizer (DDFS) to generate a desired waveform.Generally, the DDFS has been used in the telecommunication field for itshigh frequency resolution and wide frequency bandwidth. As disclosedherein, the DDFS is used to provide flexibility when generating auser-defined waveform of ultra-low frequency.

FIG. 9 is a block diagram of a DDFS 900 configured to generate auser-defined ultra-low frequency waveform according to one embodiment.The DDFS 900 includes a phase accumulator 910, a read only memory (ROM)912, a digital-to-analog converter (DAC) 914, and an analog low-passfilter 916. The phase accumulator 910 includes a binary N-bit full-adder918 and an N-bit phase register 920. The N-bit full-adder 918 includestwo inputs that are the inputs to the phase accumulator 910. The phaseregister 920 provides the output of the phase accumulator 910.

The output of the phase accumulator 910 is fed back to one of theadder's inputs. A second adder input, frequency control word (FCW), isadded to the phase accumulator on every clock cycle. A clock with afrequency fc is the synthesizer's time reference. The full-adder 918causes the contents of the phase register 920 to increase by the valueFCW at the end of each clock cycle.

The phase accumulator's output overflows to zero periodically. Theoutput of phase accumulator 910 is a sampled saw-tooth wave with arepetition frequency

${f_{out} = {f_{c} \times \frac{FCW}{2^{N}}}},$and an output frequency resolution

${\Delta\; f} = {\frac{f_{c}}{2^{N}}.}$From these equations, it is clear that f_(c), FCW, and the length ofphase accumulator 910 control the output frequency and the frequencyresolution. For example, if f_(c) is 5 kHz, FCW is 1 Hz, and the phaseaccumulator 910 has a length of 16-bits, then the output frequency islower than 0.1 Hz and will satisfy the SWV specification. If the FCWchanges, the amount of accumulated phase will change irregularly. Thismay be considered as a phase jitter in the output signal. Thus, in oneembodiment, the FCW remains unchanged.

The output of the phase accumulator 910 forms the address of the ROMbased look-up table (LUT). The size of the ROM 912 grows exponentiallywith the width of the phase accumulator 910. However, in certainembodiments it is desirable to use a wide phase accumulator 910 toachieve fine frequency resolution. This controversy may be neglected insome embodiments because a 16-bit phase accumulator is sufficient forthe 0.1 Hz output frequency. The output signal of the DAC 914 followedby the anti-alias low-pass filter 916 is a pure signal (e.g., free ofharmonic distortion). In such embodiments, the Nyquist condition:f_(c)>2×f_(out) is satisfied.

1. Example DDFS Circuit Implementation

FIG. 10 is a block diagram of an example DDFS circuit 1000 according toone embodiment. The example DDFS circuit 1000 includes an 8-bitmicrocontroller 1010, a DAC 1212, and a low-pass filter (LPF) 1014. Inthis example embodiment, the microcontroller 1010 includes a Z8 EncoreXP F082A microprocessor available from ZiLOG, Inc. of San Jose, Calif.The F08XA series devices support 8 KB of flash program memory and 1 KBregister RAM. The flash in-circuit programming capability allows forfaster development and program changes in the field. The F08XA devicesinclude rich peripherals that make them very suitable for the lowfrequency DDFS embodiments disclosed herein.

The F08XA devices provide up to 20 MHz system clock. In conventionalDDFS designs, there is generally a separate field-programmable gatearray (FPGA)/MCU to finish the DDFS due to the consideration offrequency accuracy. In this example embodiment, however, the targetfrequency is very low and processing speed is around 10 MIPS. Thus, noextra chips (FPGAs/MCUs) are needed. The F08XA devices also provide: an8 KB flash memory with in-circuit programming capability and 1 KBregister RAM; 25 general purpose input/output (IO) pins (28 pin package)wherein each pin is individually programmable; an internal precisionoscillator, which requires no external components; a full-duplexuniversal asynchronous receiver/transmitter (UART) capable of handlingasynchronous data transfers; two enhanced 16-bit timers that may be usedfor timing/counting events; an on-chip debugger (OCD) that provides arich set of debugging capabilities such as reading and writing register,programming flash memory, and setting breakpoints; a single pininterface that provides communication to the OCD; an 8-channel, 10-bitDAC; up to 20 vectored interrupts that include eight internal peripheralinterrupts and twelve general-purpose IO (GPIO) pin interrupt sourcesand have three levels of programmable interrupt priority; and power-onreset functionality.

In this example embodiment, the DAC 1012 includes an AD7392analog-to-digital converter (ADC) available from Analog Devices, Inc. ofNorwood, Mass. The AD7392 ADC is a general purpose 12-bit,voltage-switched, laser-trimmed parallel ADC. The voltage switched R-2RDAC generates an output voltage dependent on the external referencevoltage connected to the REF pin according to following equation:

$V_{out} = {V_{REF} \times {\frac{D}{2^{N}}.}}$

In one embodiment, V_(REF) is approximately 2.5V and N=12, so that:

$V_{out} = {V_{REF} \times {\frac{D}{4096}.}}$

The output of the DAC 1012 may include some unwanted high frequencynoise. The LPF 1014 is configured to reduce or remove this noise. Inthis example embodiment, the LPF 1014 includes a MAX280 5^(th) order LPFavailable from Maxim Integrated Products, Inc. of Sunnyvale, Calif. TheLPF 1014 uses an external resistor and capacitor to isolate the MAX280integrated circuit from the DC signal path to provide excellent DCaccuracy. The resistor and capacitor, along with an on-chip 4^(th)-orderswitch capacitor filter, forms a 5^(th)-order LPF. The cutoff frequencyis set by an internal clock that may be externally driven. The clock tocutoff frequency ratio in one embodiment is 100:1.

The desired cutoff frequency and the resistor and capacitor should bechosen such that:

$\frac{f_{cut}}{1.62} = \frac{1}{2\pi\;{RC}}$f_(cut): filter cutoff frequency (−3 dB point).

If f_(cut)=30 Hz, R=15 k, C=560 pF, filter gain is given by:f _(in)=0.5f _(cut) G _(filter)=−0.02dB;f _(in) =f _(cut) G _(filter)=−3 dB;f _(in)=2f _(cut) G _(filter)=−30 dB;f _(in)=4f _(cut) G _(filter)=−60 dB.

FIG. 11 schematically illustrates an example waveform 1100 generated bythe example DDFS circuit 1000 shown in FIG. 10 according to oneembodiment. In this example, the DDFS circuit 1000 synthesizes thewaveform 1100 as the combination of a triangle wave having a frequencyof approximately 0.1 Hz and a square wave having a frequency ofapproximately 15 Hz. FIGS. 12-13 schematically illustrate enlargedversions of the waveform 1100 shown in FIG. 11 to provide additionaldetails.

FIG. 14 is a schematic diagram of the example DDFS circuit 1000 shown inFIG. 10 according to one embodiment. FIG. 14 shows detailedinterconnections between the example Z8 F082A microprocessor 1010, theAD7392 ADC 1012, and the MAX280 LPF 1014 discussed above.

III. EXAMPLE CONTROL AMPLIFIER EMBODIMENTS

The control amplifier 214 (see FIG. 2) is configured to maintain apotential between the working electrode 116 and the reference electrode114 of the of the electrochemical cell 100 (see FIG. 1) at a presetvalue. The control amplifier 214 determines whether current is necessaryto flow between the working electrode 116 and the counter electrode 112so as to keep the preset potential, as long as the cell voltage andcurrent do not exceed the compliance limits of the potentiostat. If acurrent passes through the reference electrode 114, the referenceelectrode 114 will be polarized such that the potential of the referenceelectrode 114 varies with the current. Thus, to maintain a stablepotential between the reference electrode 114 and the working electrode116, no current (with the exception of possible leakage currentdiscussed below) is allowed to pass through the reference electrode 114.According to one embodiment, the control amplifier 214 therefore has avery high input impedance in a range between approximately 10¹² Ohms andapproximately 10¹⁵ Ohms.

The control amplifier 214 is configured to perform at least thefollowing three tasks: (1) measure and maintain the potential betweenthe reference electrode 114 and the working electrode 116 at a presetvalue; (2) offer enough current flow from the counter electrode 112toward the working electrode 116 in order to counteract the differencebetween the preset potential value and an existing working electrodepotential; and (3) offer high input impedance at the reference electrode114, preventing it from polarizing.

FIG. 15 is a block diagram of a control amplifier 214 according to oneembodiment. The control amplifier 214 includes an operational amplifier1510 electrically coupled to the counter electrode 112. FIG. 15illustrates the impedance between the electrodes 112, 114, 116 as Z1 andZ2. Because the input impedance of the operational amplifier 1510 ishigh, substantially no current will flow from the inverting input to thenoninverting input. Thus, the potential of the inverting input issubstantially equal to the noninverting input, which is grounded.

A predetermined reference voltage, V_(RE)=−V_(i) (with reference to theworking electrode 116), is a fixed value and does not change with thefluctuation of Z1 or Z2. The output voltage V_(o) (into the counterelectrode 112) is also a fixed value and is give by (with reference theworking electrode 116):

$V_{o} = {{- V_{i}}{\frac{Z_{1} + Z_{2}}{Z_{2}}.}}$

Thus, the potential between the reference electrode 114 and the workingelectrode 116 is fixed and equal to a predetermined input voltage V_(i).

The basic control amplifier circuit shown in FIG. 15 lacks an additionalinput channel for a control signal. Further, the reference electrode 114must supply a significant current to the summing point, which may causethe reference electrode 114 to polarize. Further, the power that isavailable at the cell 100 may only include power that is available fromthe output of the operational amplifier 1510.

FIG. 16 is a block diagram of a control amplifier 214 according toanother embodiment. The control amplifier 214 shown in FIG. 16 includesfour operational amplifiers IC01, IC02, IC03, PB. The operationalamplifier IC01 is configured as a pre-amplifier and the operationalamplifier PB is configured as a power booster. Increased power isachieved by inserting power boosting amplifier PB in the output loop.The power boosting amplifier (PB) is configured as unit gain currentamplifier capable of delivering higher currents or higher voltages thanthe operational amplifier IC01. The power boosting amplifier (PB)follows the pre-amplifier (IC01) to handle a large working current andprovides DC-DC coupling. The output potential is equal to the DCpotential of the pre-amplifier (IC01).

The operational amplifier IC02 is configured as a voltage followerinserted into the feedback loop. The voltage follower (IC02) works as animpedance transformer to obtain high input impedance. This prevents thereference electrode 114 from polarizing. The voltage follower (IC02)also provides summing current without influencing the potential of thereference electrode 114.

The operational amplifier IC03 is configured as an inverting voltageconverter with unit gain. The output voltage the inverting voltageconverter (IC03) may be measured and recorded as the potential value ofthe working electrode 116.

A. Example Control Amplifier Circuit Implementation

FIG. 17 is a schematic diagram of an example control amplifier circuit214 according to one embodiment. The control amplifier 214 includesoperational amplifiers 1710, 1712, 1714, 1716, 1718. In one embodiment,the operational amplifier 1716 includes an ultra-low bias current DIFEThigh performance operational amplifier available from Burr-Brown Corp.of Tucson, Ariz. The fabrication of the DIFET 1716 may eliminateisolation-junction leakage current. The DIFET's input impedance ofcommon-mode reaches approximately 10¹⁵ Ohms according to one embodiment.In one embodiment, the operational amplifier 1718 includes an highvoltage, high current operational amplifier, also available fromBurr-Brown Corp.

Ultra-low input bias current operational amplifiers may requireprecautions to achieve best performance. Leakage current on the surfaceof a circuit board, for example, may exceed the input bias current ofthe amplifier. To minimize surface leakage, a guard trace shouldcompletely surround the input terminals and other circuitry connectingto the inputs of the operational amplifiers. In this example embodiment,two pins next to the input pins have no internal connection. This allowsan optimized circuit board layout with guarding. The shielding layer ofa probe cable of the reference electrode 114 may also need guarding.

The operational amplifier 1710 provides DC offset of SWV. The DAC AD7392discussed above is powered by a positive voltage source, but accordingto the requirement of SWV, the sweep wave ranges from approximately−0.1V to approximately 0.6V. Thus, the operational amplifier 1710according to one embodiment is used to draw DC potential below a zerovalue. The operational amplifier 1712 is configured as a summingoperational amplifier that sums the SWV, the reference signal from thelock-in amplifier 222, and feedback signal together. The operationalamplifier 1714 is configured as a 1:−1 voltage converter. Theoperational amplifier 1714 also provides voltage gain needed for the ADCcircuit discussed above.

IV. EXAMPLE CURRENT AMPLIFIER EMBODIMENTS

As discussed above, the current amplifier 218 (see FIG. 2) is configuredto transform the ion solution induced current into a voltage value,which is more convenient to measure. The current amplifier 218 measuresthe induced current at the end of each half-cycle. The current measuredin the reversed half cycle is subtracted from the current measured inthe forward half-cycle. This current difference is displayed accordingto one embodiment as a function of the applied potential.

In one embodiment, the maximum current value produced by SWV isapproximately 2 mA. The actual induced current depends on the electricalactivity of the ion solution 110. The usable span of current betweenfull range and the lowest detectable current in the same range is calledthe potentiostat's dynamic. In one embodiment, the dynamic of thepotentiostat spans four decades. For example, in one embodiment, thedynamic ranges from approximately 100 pA to approximately 2 mA.

Because the input impedance of the reference electrode 114 is very high,the current flow through the counter electrode 112 is almost the same asthe current flow through the working electrode 116. As discussed below,the difference among various current measuring methods depends on thedifferent placement of a range resistor Rr.

A. Basic Measuring Circuit

FIG. 18 is a block diagram of a basic measuring circuit 1800 accordingto one embodiment. In order to plot the current difference versesapplied potential, the current flow through the counter electrode 112and the working electrode 116 is measured. As shown in FIG. 18, onebasic circuit for measuring the current includes a range resistor Rr inseries with the counter electrode 112 across which a voltage developsproportional to the current passing through the range resistor Rr. Inthis example embodiment, the working electrode 116 remains on trueground and the circuit is not sensitive to noise. However, in thisembodiment, a voltage meter 1810 requires a floating input, which isdifficult to implement.

B. Floating Power Supply Circuit

FIG. 19 is a block diagram of a measuring circuit 1900 including avoltage meter 1910 and a floating power supply 1912 according to oneembodiment. The circuit 1900 includes two grounds electrically coupledthrough a range resistor Rr. The first ground is a signal ground towhich the working electrode 116 is directly connected. The second groundis a power ground. When ion current flows from the working electrode 116to the power ground, a voltage develops in the range resistor Rr that ismeasured by the grounded voltage meter 1910. The zero point of the powersupply system kept on the working electrode potential by, for example,creating a virtual zero potential using a voltage follower.

The circuit 1900 allows the working electrode 116 to remain on trueground and the circuit 1900 is not sensitive to noise. The circuit 1900also advantageously provides a grounded voltage meter 1910.

C. Active Current Shunt Circuit

FIG. 20 is a block diagram of a measuring circuit 2000 that provides anactive current sink according to one embodiment. The active current sinkis a current-voltage converter that measures the current passing througha shunt. Very high-ohmic resistors may be used as the range resistor Rr.Thus, very low current can be precisely measured. The circuit 2000 alsoadvantageously provides a voltage meter that refers to ground. However,the working electrode 116 is not really grounded and the circuit 2000may be sensitive to noise.

D. Cable Selection to Avoid Triboelectric Noise Current

Generally, triboelectric current is generated in a cable by chargescreated at an interface between a conductor and an insulator due tofriction. Free electrons rub off the conductor and create a chargeimbalance that causes a current flow. Piezoelectric currents aregenerated when mechanical stress is applied to certain insulatingmaterials. These currents are generated in ceramics and othercrystalline materials used for insulating and material interconnection.Similar stored charges occur in many plastics.

In one embodiment, STFF-46 low noise coaxial cable available fromShanghai Jin ER Wire and Cable Co., LTD. of Shanghai, China is used asprobe cables and for interconnection wire within the low currentamplifier 218. The noise of this coaxial cable is approximately 4 to 5scales lower than ordinary coaxial cable.

E. PCB Clearing

Noise currents also arise from electrochemical effects. For example,commonly used epoxy printed circuit boards may generate currents ofseveral nanoamperes when not thoroughly cleaned of etching solution,flux or other chemicals and moisture. To reduce or prevent these errorcurrents, all interconnecting circuits should be thoroughly cleaned andthen be allowed to dry completely.

F. Guarding

High impedance paths between low current conductors and voltages sources(e.g., the path through the reference electrode 114) can causesignificant leakage currents. Thus, in one embodiment, leakage currentsthrough the reference electrode 114 and other high impedance paths arereduced or eliminated through guarding. Guarding uses a conductor at thesame potential as the sensitive current path to totally surround theinput leads carrying the high impedance signals. Guarding drasticallyreduces leakage currents from those leads.

For example, FIG. 21 is a schematic diagram of a circuit 2100 withoutguarding used to measure a current according to one embodiment. Thecircuit 2100 in this example includes an ammeter 2110 configured tomeasure a current I_(target) flowing through the points A-B-C-D. Becauseof the existence of leakage current I_(leakage), the total currentI_(total)=I_(target)+I_(leakage). If, for example, the potential ofpoint B is 5V and R9=10⁹ Ohms, then the leakage current I_(leakage) willbe 5 nA. If the target current I_(target) is on the order of or lessthan 5 nA, it will be overwhelmed by the leakage current I_(leakage). Inone embodiment, this problem is solved by increasing the leakageresistance R9 to approximately 10¹² Ohms, which reduces the leakagecurrent I_(leakage) to approximately 5 pA. In some applications, 5 pAstill may be large compared to the target current I_(target).

FIG. 22 is a schematic diagram of a circuit 2200 with guarding used tomeasure a current according to one embodiment. The circuit 2200 isconfigured to reduce the effects of leakage currents by guarding theconnection between the point B and the ammeter 2110. This portion of thecircuit 2200 is completely surrounded by an electrical conductor 2210connected to the same potential as that of point B. The resistance R9shown in FIG. 21 is represented in FIG. 22 as resistances R10 and R11.In a coaxial cable, for example, R10 may represent a resistance betweenthe cable's insulator and the cable's conductor, and R11 may representthe resistance between the cable's insulator and signal ground.

As shown in FIG. 22, the node between the resistance R10 and theresistance R11 is electrically coupled to the electrical conductor 2210.Because the electrical connector 2210 is also coupled to point A(through the ammeter 2110), the node between the resistances R10 and R11has approximately the same electric potential as the point A. Thus, thepotential difference across the resistance R10 is substantially equal tothe voltage burden of the ammeter 2110. Because the typical feedbackammeter 2110 has a maximum voltage burden of approximately 200 μV, theleakage current may be reduced by approximately four decades. Assuming,for example, that the resistance R10 has the same value as that ofresistor R9 above (e.g., approximately 10⁹ Ohms), the leakage currentI_(leakage01) through the resistance R10 is reduced to approximately 0.2pA.

The resistance R11, is coupled between the node (e.g., the cable'sinsulator) and the point C (e.g., the signal ground) to complete theleakage path. Again, assuming that the resistance R11 is equal to theresistor R9 above (e.g., approximately 10⁹ Ohms) and the potential ofpoint B is 5V, a leakage current I_(leakage02) flowing through theresistance R11 is still approximately 5 nA. However, the leakageI_(leakage02) is now supplied by a low impedance source (e.g., the 5Vsource) and is not a problem because it is not measured by the ammeter2110.

Guarding may be provided, for example, in coaxial cables. When a probecable, for example, has a high input impedance R_(s) compared to aleakage resistance R, and the shielding layer is grounded, the measuredvoltage is:

$\frac{R}{R + R_{S}} \times {V_{i}.}$

When the shielding layer is connected to a low impedance source of thesame potential as the high impedance source V_(i), leakage current fromthe center conductor to the shield will vanish or be reduced. Then, themeasured voltage will be approximately equal to V_(i).

G. Example Current Amplifier Circuit Implementation

FIG. 23 is a schematic diagram of an example current amplifier circuit218 according to one embodiment. The circuit 218 is controlled by theexample Z8 F082A microprocessor 1010 discussed above. The currentamplifier circuit 218 is also electrically connected to the MAX280 LPF1014 discussed above.

To meet the dynamic range, the current amplifier circuit 218 includes atwo stage current follower including the operational amplifiers 2312,2314, 2316, 2318. The gain of a first stage of the current follower maybe adjusted by the microcontroller 1010. In one embodiment, the maximumgain of the first stage is selectively set to approximately 80 dB andthe gain of the second stage is unadjustable (e.g., set at approximately60 dB). The fixed gain of the second stage may be based on the specificion solution 110 being tested. In one embodiment, the total dynamicrange is between approximately 60 dB and approximately 180 dB, includingapproximately 40 dB of fixed DC gain provided by the lock-in amplifier222 (discussed in detail below).

As shown in FIG. 23, the current amplifier circuit 218 may be connectedto the microcontroller 1010 through a MAX5160 linear-taper digitalpotentiometer 2310 available from Maxim Integrated Products, Inc. ofSunnyvale, Calif. The MAX5160 potentiometer 2310 performs the samefunction as a mechanical potentiometer or a variable resistor. TheMAX5160 potentiometer 2310 includes a fixed resistor and a wiper contactwith 32 tap points that are digitally controlled by the microcontroller1010. The MAX5160 potentiometer 2310 is configured to selectively adjustthe gain of the current amplifier 218.

V. EXAMPLE LOCK-IN AMPLIFIER EMBODIMENTS

The lock-in amplifier 222 (see FIG. 2) is configured to remove or reducenoise from a target signal to be measured. A high speed digital clock(e.g., used by the DDFS) and other circuit components may introducebroadband noise. The measured target signal is the ion current flowingthrough the electrochemical cell 100, which may be weak enough tosometimes be buried completely by the broadband noise. Generally, thedynamic of the target current will reach approximately 4 decades, fromapproximately 100 pA to approximately 2 mA.

FIG. 24 is a block diagram of a lock-in amplifier 222 according to oneembodiment. The lock-in amplifier 222 includes an input amplifier 2410configured to receive a signal input, a band-pass filter 2412, a phasesensitive detector (PSD) 2414, a low-pass filter 2416, a referencetrigger 2418 configured to receive a reference input, a phase shifter2420, and an output DC amplifier 2422 configured to provide a DC signaloutput. An active current shunt acts as a current to voltage converterthe current of the signal input (e.g., on a pA scale) is pre-amplifiedand transformed to a voltage (e.g., on the nV scale) on the DC signaloutput.

Assume, for example, that X(t) is a period target signal S(t) mixed witha random noise signal N(t) where X(t)=S(t)÷N(t)=A sin(ωt+φ)+N(t). Thereference input signal is given by Y(t)=B sin ω(t+τ), where τ is a timeoffset. A correlations function R_(xy) of mixed signal X(t) andreference signal Y(t) is

$R_{XY} = {{\lim\limits_{T->\infty}{\frac{1}{T}{\int_{0}^{T}{B\;\sin\;{{\omega\left( {t + \tau} \right)}\left\lbrack {{A\;{\sin\left( {{\omega\; t} + \varphi} \right)}} + {N(t)}} \right\rbrack}{\mathbb{d}t}}}}} = {{{\frac{AB}{2}{\cos\left( {{\omega\;\tau} + \varphi} \right)}} + {R_{NY}(\tau)}} = {\frac{AB}{2}{{\cos\left( {{\omega\tau} + \varphi} \right)}.}}}}$

Because the noise signal N(t) is non-correlated with the referencesignal Y(t), R_(NY)(τ)=0. Thus, is a DC value and proportional to theamplitude of the target signal input and the reference signal input.

In one embodiment, the PSD 2414 comprises a demodulator. The PSD 2414operates by multiplying two signals together. For example, assume thatthe target input signal is U_(s)=E_(s)×sin(2×π×f₁×t=Φ₁) and that thereference input signal is U_(r)=E_(r)×sin(2×π×f₂×t+φ₂), the PSD 2414 isconfigured to multiply these two signals to provide

$U_{out} = {{U_{s} \times U_{r}} = {{\frac{E_{s}E_{r}}{2}{\cos\left\lbrack {{2{\pi\left( {f_{1} - f_{2}} \right)}t} + \left( {\varphi_{1} - \varphi_{2}} \right)} \right\rbrack}} - {\frac{E_{s}E_{r}}{2}{{\cos\left\lbrack {{2{\pi\left( {f_{1} + f_{2}} \right)}t} + \left( {\varphi_{1} + \varphi_{2}} \right)} \right\rbrack}.}}}}$

The output of the PSD 2414 includes two parts. One part is a frequencysum of the target signal input and the reference input signal. The otherpart is a frequency difference of the target signal input and thereference input signal. When f₁=f₂, the difference part converts to a DCcomponent of the PSD's output, and the sum part has exactly twice thefrequency of the input signal, which may be isolated by the low-passfilter 2416.

In one embodiment, a symmetrical square wave may be used as thereference input signal. A Fourier series representation of the referenceinput signal may be given by

$U_{r} = {\frac{4}{\pi}{\sum\limits_{n = 0}^{\infty}{\frac{1}{{2n} + 1}{{\sin\left\lbrack {\left( {{2n} + 1} \right)\left( {{2\pi\; f_{2}} + \varphi_{2}} \right)} \right\rbrack}.}}}}$The output of the PSD 2414 may be given by

$U_{o} = {\frac{2E_{s}}{\pi}{\sum\limits_{n = 0}^{\infty}{\frac{1}{{2n} + 1}\cos{\left\{ {{2{\pi\left\lbrack {f_{1} \pm {\left( {{2n} + 1} \right)f_{2}}} \right\rbrack}t} + \left\lbrack {\varphi_{1} \pm {\left( {{2n} + 1} \right)\varphi_{2}}} \right\rbrack} \right\}.}}}}$

If f₁=f₂,

${U_{o} = {\frac{2E_{s}}{\pi}{\sum\limits_{n = 0}^{\infty}{\frac{1}{{2n} + 1}{\cos\left\lbrack {\varphi_{1} \pm {\left( {{2n} + 1} \right)\varphi_{2}}} \right\rbrack}}}}},$assume θ=φ₁−φ₂, and with the help of the low pass filter 2416,

${U_{o} = {\frac{2E_{s}}{\pi}\cos\;\theta}},$then the amplitude is given by

${U_{s}} = {\frac{U_{o}\pi}{2\;\cos\;\theta}.}$

The above discussion is based on the case of a noise-free input signal.If noise is presented on the input signal, which has no fixed frequencyor phase relationship with the reference input, it is also multiplied bythe reference input signal in a modulator, but does not result in anychange to the mean DC level. Thus, the combination of the PSD 2414 andthe low pass filter 2416 allows signals to be measured even whenaccompanied by significant noise.

A. Example Lock-in Amplifier Circuit Implementation

FIG. 25 is a schematic diagram of an example lock-in amplifier circuit222 according to one embodiment. The circuit 222 includes an AD630 highprecision balanced modulator/demodulator 2510 available from AnalogDevices, Inc. of Norwood, Mass. The AD630's signal processingapplications include phase detection and may be used to realize alock-in amplifier with working frequency up to several hundredkilohertz. The AD630 may be thought of as a precision operationalamplifier with two independent differential input stages and a precisioncomparator which is used to select an active end.

A commonly used application of the AD630 is the balancedmodulator/demodulator. By setting R_(B)/R_(F)/R_(A), the AD630 providesprecise symmetric gain of ±1 dB and ±2 dB. The balancedmodulator/demodulator topologies accept two inputs, a signal inputapplied to the amplifying channels, and a reference input applied to thecomparator. The modulated signal is the output voltage signal of activecurrent shunt, which includes background noise and the ion currentsignal. The reference signal comes from a 0.5 Hz oscillator. Thisreference signal is also added to the summing circuit of the controlamplifier 214. The noise modulated current-voltage signal is demodulatedsynchronously using phase information derived from the modulator, andthe result is low-pass filtering using a 2-pole simple filter, whichalso provides a DC gain of approximately 100 dB to the output. Theprecision input performance of the AD630 provides more than 100 dB ofsignal range.

VI. MEASUREMENT AND CONTROL

FIG. 26 is a block diagram of the MAC unit 210 shown in FIG. 2 accordingto one embodiment. The MAC unit 210 may include a microprocessor 1010, adigital potentiometer 2610, an analog-to-digital converter (ADC) 2612, aUART 2614, an on-chip debugger 2616, a start button 2618, adigital-to-analog converter (DAC) 2620, and a switch capacitor filter2622.

As discussed above, the measurement and control of the electrochemicalchip 200, along with other function modules such as sweep wavegenerator, time base of switch capacitor filter, and interface with ahost computer, is based on the 8-bit Zilog microprocessor 1010, Z8Encore F082A. This microcontroller 1010 incorporates a rich peripheralset and makes it suitable for a variety of applications including sensornetwork, motor control and other applications. The 28-pin packageZ8F082A supports a maximum of 25 port pins for general-purposeinput/output operation. This is convenient for the interconnection witheach function module. The interconnection with DAC unit 2620, the switchcapacitor unit 2622, and the digital potentiometer unit 2610 has beenpreviously illustrated.

A. Example Analog-Digital Unit

The on-chip ADC 2612 converts an analog input signal to its digitalrepresentation. In one embodiment, the on-chip ADC 2612 provides 10-bitresolution in signal-ended mode, 8 signal-ended analog input sources,interrupt upon conversion complete, and manual n-circuit calibrationemploying user code.

There are two channels of analog voltage that are monitored. The firstchannel includes a potential value from the control amplifier 214, whichis connected to pin2/ANA7 of the microcontroller 1010. The secondchannel includes a differential current value from the current amplifier218, which is connected to pin4/ANA3 of the microcontroller 1010. Thedifferential current value is measured at the end of each half-cycle andthe current measured in the reversed half cycle is subtracted from thecurrent measured on the forward half-cycle. In one embodiment, thesesampled potential and differential current values are sent to a hostcomputer (e.g., through an RS232 or USB interface).

B. UART Unit

In one embodiment, the on-chip UART unit 2614 is a full-duplexcommunication channel capable of handling asynchronous data transfers.The UART 2614 uses a single 8-bit data mode with selectable parity.Separate transmit and receive interrupts may be initialized.

VII. EXAMPLE MEASUREMENTS WITH CONVENTIONAL AND NEW ELECTROCHEMICALMEASUREMENT SYSTEMS

To verify the systems and methods disclosed herein, the electrochemicalchip 200 was tested using cyclic voltammetric measurements withapproximately 2 mm gold electrodes in a K₃Fe(CN)₆ solution, which is anelectrochemical standard. The test results were compared to resultsobtained from a conventional electrochemical instrument. For example,FIG. 27A schematically illustrates SWV responses to various K₃Fe(CN)₆solutions using a conventional electrochemical instrument. For thistest, the conventional electrochemical instrument included a CHI 1220electrochemical analyzer available from CH Instruments, Inc. of Austin,Tex. that was used to test 0.01 M phosphate buffered solutions (PBS, pH7.4) with various K₃Fe(CN)₆ concentrations (0.1 mM, 0.3 mM, 0.5 mM, 0.5mM, 0.8 mM, and 1.0 mM).

By way of comparison with the waveforms shown in FIG. 27A, FIG. 27Bschematically illustrates SWV responses to the same K₃Fe(CN)₆concentrations discussed n FIG. 27A by the electrochemical chip 200disclosed herein with the same 2 mm gold electrodes. As shown, the SWVcurves in FIG. 27B are very similar to the SWV curves in FIG. 27A,confirming the accuracy of the systems and methods described herein.

FIG. 27C schematically illustrates data combined from FIGS. 27A and 27Bto further illustrate the consistency of responses from conventionalelectrochemical instruments and the systems and methods disclosedherein. FIG. 27C is the comparison of the peak currents vs. theconcentrations of K₃Fe(CN)₆. In FIG. 27C, data generated by theelectrochemical chip 200 is illustrated as plus (“+”) marks and datagenerated by the conventional electrochemical instrument (CHI 1220) isillustrated as circles (“◯”). In this example, the SWV parametersettings are the same for both instruments and the data corresponds tothe same K₃Fe(CN)₆ concentrations (0.1 mM, 0.3 mM, 0.5 mM, 0.5 mM, 0.8mM, and 1.0 mM) discussed above. As shown in FIG. 27C, theelectrochemical chip 200 disclosed herein provides data that correspondsvery closely to the data provided by the conventional electrochemicalinstrument. Further, the electrochemical chip 200 provides a straightercalibration curve between approximately 0.3 mM and approximately 1.0 mMthan that provided by the conventional electrochemical instrument. Thus,the electrochemical chip 200 provides better linearity in this rangethan that provided by the conventional electrochemical instrument.

In addition to testing K₃Fe(CN)₆ solutions, the systems and methodsdiscussed herein were used to test standard solutions of varying metalsPb, Zn, and Cu using ASV. Example test results are discussed below (seeFIGS. 32-40).

VIII. EXAMPLE MICROELECTRODE ARRAY SENSOR

In one embodiment, the electrochemical chip 200 may be used with one ormore microelectrode arrays that include individualized gold workingelectrodes 116 strategically arranged in an array fashion to detectheavy metal ions in aqueous solutions. Utilizing micron sizedmicroelectrodes in a geometric array has been shown to improve masstransport, thus improving the sensitivity and selectivity of the goldmicroelectrodes. In one embodiment, these arrays have the capability torun as individualized microelectrodes while performing numerous tests ina single experimentation, or to run as one electrode increasing theoverall surface area of the electrode. In addition, or in anotherembodiment, the arrays provide some intermediate partialindividualization and partial colonization of the electrodes.

In one embodiment, the microelectrode arrays are quantified by, andproven with, electrochemical standardizing potassium ferric cyanide(PFC) solution and cyclic voltammetry (CV). Using this standard, highsensitivity, selectivity, and repeatability have been shown. Heavy metalion detection is preformed with the anodic stripping voltammetry (ASV)electrochemical test and trace levels of copper, lead, and zinc havebeen detected and standardized in aqueous solutions. Testing (discussedbelow) shows high specificity and accurate technique for heavy metal iondetection in aqueous solution.

Traditionally, heavy metal ion sensors associated with anodic strippingvoltammetry (ASV) have been composed of macro-scale electrodes (e.g.,radius of approximately 1 mm). ASV is a very accurate procedure indetermination of trace metals in aqueous solution and with theinitiation of micron sized electrode surface area, according toembodiments disclosed herein, trace amounts (e.g., parts-per-billion(ppb)) of metals can be detected in times less than approximately 3minutes. Electrochemical testing in this nature (voltammetry) onlyrequires a relationship between a potential and measured current, whichin effect contributes to the analyzing instrument's ability to be smalland compact. This improves portability of the entire testing process.Furthermore, utilizing a fabrication technology similar to that usedwith integrated circuits, these sensors may be constructed relativelyinexpensively and reproducibly with precise geometries.

By decreasing the size of the electrode surface area to a micron state,according to certain embodiments, mass transport is dramaticallyenhanced. This increase in mass transport greatly improves thesensitivity and selectivity of the electrode, thus improving thedevice's versatility. Furthermore, micron sized electrodes in arraysenable the user to compare analytes in aqueous solution at varyinggeometric locations. In effect, this allows multiple repetitions ofanalyte detection during a single iteration of testing. Simultaneously,these microelectrodes can vary in measurement range, differing among themicroelectrodes comprising the array, to detect multiple elements in onebroad test. These combined factors make multiplex detection of tracemetals at very low levels possible in highly accurate robustmeasurements.

In one embodiment, microelectrode arrays are constructed on a singleglass slide, with each array including 18 microelectrodes. In certainsuch embodiments, these microelectrodes are approximately 10 μm byapproximately 10 μm squares with approximately 100 μm distances betweenadjacent electrodes. An artisan will recognize, of course, that themicroelectrodes need not be square and may be other shapes such asrectangular, circular, or any other shape. In one embodiment,approximately 100 times the width of the electrodes is the approximateminimum distance between adjacent working electrodes in order tomaximize quantification of working electrode overpotential andimpedance. In one embodiment, a protective layer of photoresist is heatset to provide hardness masking of all circuitry of the electrodes, alsoadding to greater quantification.

In one embodiment, the microelectrode arrays (MEA) are based on a planardesign where approximately 10 μm by approximately 10 μm gold workingelectrodes are positioned (approximately 10 μm thick) on glass to formarrays. Adequate adherence to glass is accomplished by a thin(approximately 150 nm) layer of Titanium between the gold surface andthe glass support base. In this example embodiment, glass was chosen asthe support base for its low electrical conductivity.

FIGS. 28A-28B are schematic diagrams of nine microelectrodes arranged inrespective half arrays 2810, 2812 according to certain embodiments. Forillustrative purposes, FIGS. 28A and 28B are scaled differently withFIG. 28A being enlarged more than FIG. 28B. In this example, theapproximate geometry of the microelectrodes and arrays are as follows: a50 μm×50 μm base microelectrode with 30 μm wide circuitry to connect themicroelectrode with an outer slide connection pad. An array includes 18microelectrodes and a half array includes nine microelectrodes. For easein fabrication, there are three central microelectrodes in a half arraythat have short spans of 10 μm wide connection circuitry. A final layerof photoresist masks the entire slide with exception of a 10 μm×10 μmhole allowed over each 50 μm×50 μm pad and the connection pads on theglass slide edges. The half array 2810 shown in FIG. 28A is illustratedbefore the final photoresist mask is applied and treated. The half array2812 shown in FIG. 28B is illustrated after the final photoresist maskis applied and treated.

FIG. 29 is a schematic diagram of a completed array that illustratesconnection pads and interconnection traces for a first half array 2910of nine microelectrodes and a second half array 2912 of ninemicroelectrodes according to one embodiment. Although not shown in FIG.29, each half array 2910, 2912 includes the microelectrodes shown inFIG. 28B.

FIG. 30 is a schematic diagram of various steps (steps A-G) in a processfor manufacturing the microelectrode array shown in FIG. 29 usingphotolithography according to one embodiment. In step A, a glass slide3010 is provided. In step B, a thin (e.g., approximately 200 nm thick)layer of Titanium 3012 is deposited onto the glass slide 3010. Ratherthan using a conventional chrome/gold layer deposited on the glass slide3010, the layer of Titanium 3012 provides relatively stronger adhesionof a layer of gold 3014 (see step C) to the glass slide 3010. Theimproved adhesion allows a broader potential range that can be appliedto the working electrode 116. In step C, the layer of gold 3014 (e.g.,approximately 10 μm thick) is deposited onto the Titanium 3012. In stepD, a photoresist layer 3016 is deposited onto the gold 3014. In steps Ethrough G, the gold 3014 is then stripped to the geometries specifiedfor the 18 individually isolated microelectrodes using a basicphotolithography technique.

In one embodiment, the glass slides 3010 are completed with three arraysper slide 3010, giving a total of 54 individually isolated workingmicroelectrodes per slide 3010. Another step (not shown in FIG. 30)includes an adhesion of wells cut from a 96 welled plate. In oneembodiment, the wells are attached with the use of a non-conductiveepoxy that does not effect the photoresist mask 3018 or the individualcircuitry that comprises the microelectrode arrays.

FIG. 31 is a schematic diagram of a finished glass slide 3100 with theindividual microelectrodes (shown in detail in FIG. 28B) comprising thethree arrays respectively corresponding to three wells (eachschematically represented as a pair of concentric circles) according toone embodiment. Each array is respectively organized in respective halfarrays 3110, 3112, 3114, 3116, 3118, 3120 that each include ninemicroelectrodes. FIG. 31 also illustrates a numbering scheme (R1-R9 andL1-L9) developed to label each array.

IX. EXAMPLE EXPERIMENTS

The following disclosure relates to example experiments performed usingthe systems and methods disclosed herein. In these example experiments,photolithographic techniques were used to fabricate single element goldmicroelectrode arrays with 18 individually isolated 10 μm×10 μm goldmicroelectrodes in a single array. These arrays were orientated to workindividually or simultaneously on a single glass slide. Initial testswere conducted using PFC as a standard to test the array's capacity tobe used with electrochemical tests. This standard is widely utilized forcalibration of fabricated microelectrodes. CV was used as theelectrochemical test which applies a sweep potential that goes from anegative state to a positive state and then cycles back from positive tonegative. Throughout the sweep, potential current changes were monitoredand (along with standardized solutions) are used to quantify themicroelectrodes.

ASV is another electrochemical method of testing for heavy metal ions insolution. ASV is performed by applying a constant negative potential fora predetermined amount of time and then sweeping the potential towards apositive region. As the potential is being swept from negative topositive, the current changes are monitored. The sweeping mechanism mayvary. In these example experiments a differential pulse mechanism isused for sweeping. ASV that uses a differential pulse sweeping mechanismis known as DPSV.

Heavy metal ion detection is possible due to positive charged ionsattaching to the electrode surface during the initial negative potentialcharge. As the sweeping potential moves towards a positive region, theions disperse off of the electrode surface at a characteristicpotential. Each ion has a distinct characteristic potential that is usedto identify it in the solution.

In these example experiments, the microelectrode arrays are used todetect traces of Cu, Pb, and Zn ions in varied concentrations ofsolution. In one embodiment, the example experiments accurately andreproducibly detect Cu, Pb, and Zn ions at a sensitivity ofapproximately 1 ppb in an aqueous solution.

The materials and methods used in these example experiments may besummarized as: photolithography; Denton E-Beam evaporator; Electromaskpattern generator and image repeater; Suss spinner/aligner; EV 420 frontand backside aligner; and 1813 Photoresist. The electrochemicalparameters of these example experiments may be summarized as: CHI 1220electrochemical analyzer (CH Instruments, TX) and corresponding controlsoftware; 18 microelectrode Au array with Ag/AgCl referencemicroelectrodes and platinum wire counter microelectrodes; potassiumferricyanide (K3Fe(CN)6) solution in 0.01 M PBS, 18M Ωcm (DI) water, and1,000 ppm Cu reference solution (Fishersci, Inc.); and PlasmaLab MicroPreactive ion etch.

A. Experimental Results

As discussed above with respect to FIGS. 30 and 31, the process used forfabrication of the microelectrode arrays for these example experimentsis a photolithographic procedure. FIG. 30 shows a generalized procedureusing photolithography where the process is highly repeatable andaccurate to small (e.g., approximately 1-5 μm) limits. FIG. 31 shows acompleted slide and the microelectrode numbering scheme for each ofthree wells on a glass slide. These microelectrodes constitute thearrays that make up a single test site.

FIGS. 32-33 graphically illustrate results of initially testing themicroelectrodes disclosed herein in Potassium Ferric Cyanide (K₃Fe(CN)₆)using cyclic voltammetry. As shown in FIG. 33, there is a distinctlinear relationship between concentration and change in height (e.g.,current) in CV analysis. This result indicates that the microelectrodesare working well. Although not shown, the test also indicated that eachmicroelectrode produces substantially the same results at a constantconcentration.

After standardizing and quantifying the microelectrodes, an analysis inwater is performed in order to check the calibration of themicroelectrodes with known concentrations of solutions. Example testswere performed on Pb and Zn. Preliminary data was also collected on Cu.FIGS. 34-36 graphically illustrate results of testing themicroelectrodes disclosed herein in Pb ions using anodic strippingvoltammetry (ASV). Absolute values of the peak heights are tabulatedcorresponding to each concentration. As shown in FIG. 36, there is adistinct linear relationship between concentration and current.Regression statistics corresponding to FIG. 36 are given in Table 2below.

TABLE 2 Slope 3.588 Intercept −36.765 R2 0.946 Steyx 35.381

Following the measurement of the Pb ions, Zn was examined in solution.FIGS. 37-39 graphically illustrate the ASV test results for Zn ions. Asseen in FIGS. 37 and 38, there is a shift in the characteristicpotential data as Zn ion concentration is increased. The cause of thisshift has not yet been determined. As with the Pb test results, the peakheights are tabulated with corresponding concentration. As shown in FIG.39, there is a linear relationship between concentration and current.Regression statistics corresponding to FIG. 39 are given below in Table3.

TABLE 3 Slope 8.074 Intercept 31.398 R2 0.935 Steyx 96.993

Preliminary tests were also performed with Cu ions in solution. FIG. 40graphically illustrates the DPSV test results for the Cu ions. As shownin FIG. 40, the DPSV peaks at approximately 4, 12, and 50 ppb. Theseresults seem to follow the same trend as both Pb and Zn, and similarlinear relationships between peak heights and concentrations areexpected.

Based on these example experiment results, the microelectrode arrays andmethods disclosed herein were proven to be highly selective andsensitive to low concentrations by standardizing them with PotassiumFerric Cyanide solution. Using a DPSV mode of ASV, current/voltagerelationships and standard calibrations for Pb, Zn and Cu ions wereobtained. The desired linear concentration versus current change for Pband Zn are shown in FIGS. 36 and 39. Tables 2 and 3 may be used withunknown concentrations of Pb and Zn to quantify the unknownconcentrations.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

The invention claimed is:
 1. A handheld sensor device for measuring anion concentration in a solution in an electrochemical cell that includesa counter electrode, a working electrode, and a reference electrode, thehandheld sensor device comprising: a function generator configured togenerate a waveform specified by an electrochemical technique; and apotentiostat integrated with the function generator in a single,portable device, the potentiostat comprising: a control amplifierconfigured to provide a current through the counter electrode and theworking electrode so as to maintain a predetermined voltage between theworking electrode and the reference electrode; and a current amplifierconfigured to measure the current provided through the counter electrodeand the working electrode, wherein the current amplifier is configuredto measure the current throughout an entire range comprisingapproximately 100 pA to approximately 2 mA.
 2. The handheld sensordevice of claim 1, further comprising a microprocessor configured toallow a user to selectively specify one or more parameters for thewaveform generated by the function generator.
 3. The handheld sensordevice of claim 1, wherein the function generator comprises a directdigital frequency synthesizer (DDFS) comprising a phase accumulator. 4.The handheld sensor device of claim 3, wherein the phase accumulatorcomprises: an N-bit phase register controlled by a clock signalcomprising a clock frequency; and an N-bit adder comprising: a firstinput for a frequency control word from a microprocessor; and a secondinput for feedback from the N-bit phase register, wherein the contentsof the N-bit phase register increase by the value of the frequencycontrol word at the end of a cycle of the clock signal.
 5. The handheldsensor device of claim 4, wherein a frequency of the waveform isselectively controlled by the frequency control word.
 6. The handheldsensor device of claim 5, where the frequency of the waveform and afrequency resolution of the waveform are further based on the clockfrequency and the N number of bits.
 7. The handheld device of claim 3,wherein the DDFS further comprises: a memory configured to receive anoutput of the phase accumulator; a digital-to-analog converterconfigured to receive an output of the memory; and an low-pass filterconfigured to remove harmonic distortion from an output of thedigital-to-analog converter.
 8. The handheld sensor device of claim 1,wherein the electrochemical technique is selected from the groupcomprising square wave voltammetry, cyclic voltammetry, linear sweepvoltammetry, differential-pulse voltammetry, and normal-pulsevoltammetry.
 9. The handheld sensor device of claim 1, wherein thefunction generator comprises: a first Schmitt circuit configured togenerate a square wave; a second Schmitt circuit configured to generatea triangle wave; and a summing amplifier circuit configured to combinethe square wave and the triangle wave.
 10. The handheld sensor device ofclaim 9, wherein at least one of the first Schmitt circuit and thesecond Schmitt circuit comprises a constant current source.
 11. Thehandheld sensor device of claim 1, wherein the current amplifiercomprises an electrically conductive guard configured to reduce aportion of the current measured by the current amplifier that isattributable to a leakage current through the reference electrode. 12.The handheld sensor device of claim 1, wherein the control amplifier isfurther configured to prevent the reference electrode from polarizing.13. The handheld sensor device of claim 1, wherein the current amplifiercomprises a voltage meter referenced to ground.
 14. The handheld sensordevice of claim 1, wherein the current amplifier comprises apotentiometer controlled by a microprocessor to selectively adjust thegain of the current amplifier.
 15. The handheld sensor device of claim1, wherein the potentiostat further comprises a lock-in amplifiercomprising a phase sensitive detector configured to multiply a phaseshifted reference signal by the current provided through the counterelectrode and the working electrode, the lock-in amplifier configured toreduce the current's noise.
 16. A handheld sensor device of claim 1 witha single integrated electrochemical chip, the single integratedelectrochemical chip comprising: a microprocessor; said functiongenerator which is controlled by the microprocessor to selectivelygenerate a waveform for use in an electrochemical technique; and saidpotentiostat which is configured to generate and measure a current in asolution.
 17. The handheld sensor device with an integratedelectrochemical chip of claim 16, wherein the function generatorcomprises: a first Schmitt circuit configured to generate a square wave;a second Schmitt circuit configured to generate a triangle wave; and asumming amplifier circuit configured to combine the square wave and thetriangle wave.
 18. The handheld sensor device with an integratedelectrochemical chip of claim 17, wherein at least one of the firstSchmitt circuit and the second Schmitt circuit comprises a constantcurrent source.